Generally, the switching devices may be divided roughly into two types: contact-incorporated type and contactless type. The contact-incorporated type switching devices, which are called by different names such as reed switches, rubber type switches, mechanical switches, etc., depending on the kind of the element(s) used for effecting switching of the electric circuit, involve some undesirable problems in use, such as chattering, variation of contact resistance, fusing of the contacts, etc.. Therefore, the contactless switching devices, which are free of such troubles, are prevalently used in applications where high reliability is required.
Heretofore, the contactless switches utilizing the so-called hall effect have been popularly used, but they still have some serious problems such as high cost of the intergrated amplification circuit including the "hall elements" and high power consumption.